The present invention relates to an electric motor of wide variable speed range and large load fluctuation, and a control system for such a motor.
A speed control apparatus for an electric motor is known from the laid open Japanese Patent Application No. 193986/1982 assigned to the same assignee, and, is shown in FIG. 1. This apparatus has the function of making a steady-state error (.DELTA.C.sub.n) equal to zero with a comparatively simple arrangement.
Referring to FIG. 1, Cs(S) indicates the disired r.p.m. of the motor N(S) the control value for determining the r.p.m. of the motor, G(S) the forward transfer function of the control system, D(S) a disturbance to the system, Y(S) the transfer function of the disturbance, and H(S) the feedback transfer function. Blocks .alpha. and 1/S indicate an integral transfer function. Then, the following holds: EQU N(S)=G(S) (1+.alpha./S) .DELTA.C.sub.n (S)+Y(S) D(S) (1) EQU C.sub.n (S)=H(S) N(S) (2) EQU .DELTA.C.sub.n (S)=C.sub.s (S)-C.sub.n (S) (3)
From Eqs. (1), (2) and (3), the steady-state error .DELTA.C.sub.n becomes: ##EQU1## where C.sub.s (S)=C.sub.s /S and D(S)=d/S. Thus, the steady-state error .DELTA.C.sub.n (hereinafter, termed the "r.p.m. fluctuation") can be made zero.
It is accordingly possible to eliminate r.p.m. fluctuations ascribable to:
(1) dispersion in the load of a mass-produced product, PA1 (2) dispersion in a motor output, PA1 (3) voltage fluctuation, and PA1 (4) power source frequency fluctuation (especially, the difference of frequencies in 50 Hz and 60 Hz zones).
As shown by a hatched part I in FIG. 2(a), however, when the target value (1) of the motor r.p.m. has changed abruptly to a lower target value (2); for a certain time interval the r.p.m. becomes too small due to a slow resonse of the control system.
Besides, in a case where the motor is to be stopped at a predetermined position, it is first set to operate at a low speed, and subsequently performs a stopping operation. However, when a sudden and large load fluctuation occurs during the low speed operation, the r.p.m. becomes too great as shown by a hatched part II in FIG. 2(a), and a stop position can shift widely.